This invention relates to a communication resource allocation method and apparatus for allocating signals of plural users in a predetermined band for transmission.
Recently, satellite transmission, mobile transmission or the like uses a communication resource allocation method wherein a plurality of earth station or subscriber station join a single transponder or base station for mutual exchange of information for communication. For example, in communication resource allocation for mobile transmission, a single base station is commonly utilized by a plurality of mobile stations (users). Thus, a variety of methods have been conceived to avoid an interference between respective mobile stations. As methods for this purpose, frequency division multiple access (FDMA), time division multiple access (TDMA) and code division multiple access (CDMA) are currently available.
Of these methods, the CDMA is one communication resource allocation method in which a particular code is allocated to each of mobile stations, a modulated wave by the same carrier is spread in the form of spectrum by this code and transmitted to the same base station and a receiving side encodes respective waves by synchronizing to identify a desired mobile station.
That is, the base station occupies all the band by spread spectrum and transmits to respective mobile stations in the same interval of time and by use of the same frequency band. Then, each mobile station de-spreads signals having a fixed spread band width transmitted from the base station to fetch an appropriate signal. Additionally, the base station identifies respective mobile stations by mutually different codes for spreading.
This CDMA makes it possible to communicate by each direct call if the particular code is determined between the base station and each the respective mobile stations. Additionally, the CDMA is excellent in secrecy in communication and therefore suitable for radio transmission using a mobile station, such as a cellular phone or the like.
By the way, the CDMA makes it difficult to place signals transmitted from different mobile stations in strict orthogonal relationship, so that they cannot be separated from each other completely, thereby other mobile stations being an interfering source. Further, if an application band width is defined, the other band widths cannot be applied.
For example, FIG. 1 illustrates a model for extracting, by de-spreading, a particular user""s signal from eight mobile station (user) signals multiplexed by coding. If it is intended to extract, by de-spreading, U0 from eight user signals U0-U7 multiplexed by coding, although user signal U0 can be extracted, the other seven user signals U1-U7 handled by the same base station becomes an interfering source. As a result, as shown in FIG. 1B, noise from the other signals U1-U7 ride on the signal U0, thereby S/N characteristics deteriorating. Thus, radio transmission using the CDMA has a narrow service area because radio wave transmission lowers due to deterioration by interference. Further, because an interference by the other users can be suppressed only by a spreading gain obtained in a process of spectrum de-spreading, the user (mobile stations) capable of connecting to the base station is limited so that the capacity of channel is reduced.
The spreading band width is usually fixed and the number of users which can be multiplexed is limited, therefore the CDMA cannot flexibly cope with respective conditions in frequency allocation different depending on countries. Thus, only a relatively narrow band width can be defined, so that a maximum user rate is also limited.
To solve the problem described above, it is an object of the present invention to provide a communication resource allocation method and apparatus wherein separation of signals among respective users can be achieved completely so as to prevent deterioration of such characteristics as S/N or the like, the number of users which can be multiplexed can be secured to its maximum extent depending on the band width and transmission rate can be changed.